1. Field of the Invention
The invention relates to a device for transmitting optical signals between units rotatable relative to each other. Devices of this kind are preferably used in computer tomographs.
2. Description of the Prior Art
Various devices are known for transmitting optical signals between units that are rotatable relative to each other, particularly with an unobstructed inner diameter. Here a basic problem consists of designing a means for carrying light along the circumference of the device, and also suitable means for coupling light in or out. For application in computer tomographs, devices of this kind must have large unobstructed inner diameters of a magnitude of 1 meter. During rotation, the circumferential speed may be of a magnitude of 20 m/s. At the same time, data rates of more than 1 gigabyte per second (gigabaud) should be feasible.
Thus, the U.S. Pat. No. 4,109,997 discloses an optical rotary data transmission device in which the light is carried along the circumference by reflection at two opposite surfaces. Light guides or glass fibers are provided for coupling light in or out, a bundling or focusing of the light beam being effected by means of lenses. However, this device has a whole series of disadvantages. Thus, an optical transmission loss is comparatively large owing to multiple reflections at relatively steep angles. Thus, high transmitting powers are needed in an optical transmitter. Furthermore, the fabrication costs are relatively high, because of the oppositely disposed mirror-finished surfaces. Wideband data transmission with period lengths of the modulation signal which are substantially shorter than the transit time of the signal around the circumference of the device is not possible, because signals are received along multiple-paths at positions of the receiver close to the transmitter. Thus, signals received along a short path from the transmitter, and signals which have been reflected at least once around the circumference of the device, are received simultaneously. The transit time difference must be small compared with the period length of the modulation signal. Thus, with an inner diameter of about one meter, a total transit time of about 10 nanoseconds results. With this, for example, bit periods of maximally 50 nanoseconds, corresponding to a maximum transmission rate of 20 megabaud, can be achieved during a transmission of digital signals
An improvement of the optical system is disclosed in the U.S. Pat. No. 4,525,025. In this, FIG. 10 particularly illustrates a specially suitable trench for a transmission of optical signals. This consists of only one part, and therefore can be manufactured at low cost. However, even this patent specification sets out no effective solution of the problem of bandwidth limitation. Thus, this device is suitable only for small diameters.
An improvement of optical coupling in or out is disclosed in the U.S. Pat. No. 4,555,631. In this, a coupling in of optical signals is effected in a mirror-coated cylinder by means of two mirrors. For coupling out, an additional coupling-out element is provided and disposed at a fixed position in the trench. Here too, the problem of bandwidth limitation has not been solved. Thus, the light is passed from a coupling-in position to a coupling-out position along two paths in opposite directions, and finally jointly evaluated in a receiver. Here too the limitation applies that the period length of the modulation signal must be substantially shorter than the transit time of the light around the circumference of the device.
In order to reduce the attenuation by a transmission path, and to increase the transmittable bandwidth, a transmission along a plurality of shortened segments is proposed in the U.S. Pat. No. 6,104,849. The shortened segments result in a reduced attenuation. Here, the maximum bandwidth is inversely proportional to the length of the segments. Thus, a larger bandwidth may be achieved with shorter segments. For this, however, a correspondingly higher number of optical transmitters or receivers is needed to cover the full circular circumference. Therefore the system costs rise in proportion to the bandwidth.
DE 195 43 386 C1 describes in general form a device for wideband signal transmission which, although making possible a large bandwidth, provides no indications of a transmission with high transmission quality. The contents of DE 195 43 386 C1 are incorporated in the present document by reference.
In WO 03/069392 a device is disclosed for wideband signal transmission by means of a light guide that is divided along the longitudinal direction.